如何在Switch語句中使用IF

Question

如何應用if語句中的條件，我想計算Average：但是我試過最好解決問題但仍然沒有從Switch語句輸出。 我剛使用C++，

#include <iostream> 

using namespace std; 

int main() 
{ 
    //declaration of variables. 
    int sub1,sub2,sub3, sub4,sub5,total,avg; 

    //accepting marks from user in each subject 
    cout << " Enter Programming in C++ Marks: " << endl; 
    cin >> sub1; 
    cout << " Enter Software Engineering Marks : " << endl; 
    cin >> sub2; 
    cout << " Enter Personal Communication Marks : " << endl; 
    cin >> sub3; 
    cout << " Enter Database Application Marks: " << endl; 
    cin >> sub4; 
    cout << " Enter Computer Concept Marks: " << endl; 
    cin >> sub5; 

    //calculatin sum of marks obtained in each subject 
    total = (sub1 + sub2 + sub3 + sub4 + sub5); 

    //calculating the average marks 
    avg = total/5; 

    // starting of if condition for finding out grades of total subjects. 

    switch (avg){ 
     case 1:{ 
     if ((avg >= 80) && (avg <= 100)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << "You Grade is A+ " << endl; 
     } 
     break; 
     } 
     case 2:{ 
     if ((avg >= 70) && (avg <= 79)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your grade is A " << endl; 
     }  
     break; 
     } 
     case 3:{ 
     if ((avg >= 60) && (avg <= 69)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your Grade is B+ " << endl; 
     } 
     break; 
     } 
     case 4:{ 
     if ((avg >= 50) && (avg <= 59)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your Grade is C+ " << endl; 
     } 
     break; 
     } 
     case 5: { 
     if ((avg >= 40) && (avg <= 49)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your Grade is C- ! " << endl; 
     } 
     break;  
     } 
     case 6: { 
     if ((avg >= 30) && (avg <= 39)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your Grade is D ! " << endl; 
     } 
     break; 
     } 

     default:{ 
     if ((avg >= 100) && (avg <= 29)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << " Your Grade is F, So you are Fail in the class ! " << endl; 
      break; 
     } 
     } 
    } 

    system("pause"); 
} 

Answer 1

switch語句用於執行一個依賴於特定值的代碼塊。從某種意義上說，switch語句可以被認爲是一個if語句的形式：代碼

switch (avg) { 
    case 1 : { /* code block 1 */ } break; 
    case 2 : { /* code block 2 */ } break; 
    default : { /* code block default */ } break; 
} 

可以理解爲

if (1 == avg) { /* code block 1 */ } 
else if (2 == avg) { /* code block 2 */ } 
else { /*code block default */ } 

你的switch語句可以理解爲

if (1 == avg) 
{ 
    if ((avg >= 80) && (avg <= 100)) 
    { 
     cout << " Your Average is: " << avg << endl; 
     cout << "You Grade is A+ " << endl; 
    } 
} else if... 

並且沒有辦法avg可以== 1並且大於80且小於100，這就是爲什麼你沒有得到任何輸出。

在C++中，switch語句不適合測試範圍;我只是用一個if語句：

if ((avg<=100) && (avg >=80)) 
{ 
    // you get an A 
} else if ... 

但是，如果你真的需要使用一個開關，有幾個方法可以去了解它：

switch (avg) { 
    case 100: 
    case 99: 
    case 98: 
    ... 
    case 80 : { /* you get an A+ */ break; } 
    case 79 : 
    case 78 : 
    ... 
    case 70 : { /* you get a A */ break: } 
etc. 

這是醜陋的，但它是一個switch語句。使用switch語句的一種更簡潔的方法是將avg「強制」爲離散值，而不是範圍。

int percentile = avg/10; // integer division. 

現在範圍將是0和10之間，假定的平均值是0和100之間這將使一個稍微清潔器開關：

switch (percentile) { 
    case 10 : 
    case 9: 
    case 8: /* You get an A+ */ break; 
    case 7: /* You get an A */ break; 
    case 6: /* You get a B+ */ break; 

etc. 

希望這是很有幫助的。

Answer 2

您正在使用切換有點不對，這不是if問題。這：

switch (avg){ 
    case 1:{ 
     if ((avg >= 80) && (avg <= 100)) 
     { 
      cout << " Your Average is: " << avg << endl; 
      cout << "You Grade is A+ " << endl; 
     } 
     break; 

的含義一樣

if(avg == 1) 
{ 
    if ((avg >= 80) && (avg <= 100)) 
    { 
     cout << " Your Average is: " << avg << endl; 
     cout << "You Grade is A+ " << endl; 
    } 
} 

如此明顯的情況永遠不會得到滿足。對於您的特定情況，您可以取下開關，只留下if。

Answer 3

那麼，switch將case與您給它的變量進行比較。因此，對於case 1，它會在之後嘗試比較avg到1和，它將繼續與您的if聲明一起使用，當然聲明永遠不會應用。

我不確定你在那裏試着做什麼;所有我可以看到你是一個嵌套if更好：

if ((avg >= 80) && (avg <= 100)) 
{ 
    cout << " Your Average is: " << avg << endl; 
    cout << "You Grade is A+ " << endl; 
} else if ((avg >= 70) && (avg <= 79)) 
{ 
    cout << " Your Average is: " << avg << endl; 
    cout << " Your grade is A " << endl; 
} else if ((avg >= 60) && (avg <= 69)) 
{ 
    cout << " Your Average is: " << avg << endl; 
    cout << " Your Grade is B+ " << endl; 
} ... 

但即使這樣，可以更優雅的處理，這取決於你的技能。例如，您可以除以10平均值，並使用上switch，利用整數除法截斷事實上，倒塌50和59之間什麼東西都往同一個桶5：

switch (avg/10) { 
    case 10: 
    case 9: 
    case 8: 
    cout << " Your Average is: " << avg << endl; 
    cout << "You Grade is A+ " << endl; 
    break; 
    case 7: 
    cout << " Your Average is: " << avg << endl; 
    cout << "You Grade is A " << endl; 
    ... 
} 

而且你會發現，你打印»您的平均值是：...«每一次，所以你可以前條件移到該行：最後

cout << " Your Average is: " << avg << endl; 
switch (avg/10) { 
    ... 
} 

，因爲你有一個非常小的LIS可能的結果噸，可以收集它們在陣列中並簡單地引用適當的數組元素，甚至無需一個if/else if或switch：

char[][] grades = { 
    "F", "F", "F", "D", "C-", "C+", "B+", "A", "A+", "A+", "A+" 
}; 
char[] grade; 
if (avg > 100) 
    grade = "F"; 
else 
    grade = grades[max(0, avg/10)]; 

cout << " Your average is " << avg << endl; 
cout << " Your grade is " << grade << endl; 

Answer 4

當你說

Switch(avg) 

{ 
    case 1: 
    break; 
} 

它是相同如同說if(avg == 1)。所以刪除您開關的情況下，處理像這樣

if ((avg >= 80) && (avg <= 100)) 
//do this 
else if ((avg >= 70) && (avg <= 79)) 
//do that 

Answer 5

的代碼總是在默認statment去，因爲你的switch敘述將不會得到任何輸入作爲的情況下提供的。

Answer 6

像1,2,3一樣的情況被switch語句識別爲avg的值。

您只能使用帶有特定值（而非間隔）的switch（）控件。我建議你在這種情況下使用else。

或者你可以在多種情況下實現1條語句（但不會被讀取）。

switch(avg){ 
    case 80: case 81: case 82: //do this until 100 

     cout << " Your Average is: " << avg << endl; 
     cout << "You Grade is A+ " << endl; 
     break; 
} 

而且您不需要爲每種情況使用範圍（大括號）。

Answer 7

最好也許最簡單的做法是擺脫switch語句並使用「if」和「elseif」。

Answer 8

刪除Switch- case，你將得到你想要的輸出，因爲在這種情況下，它總是處於默認狀態。